With the increased image quality and reduced cost, monitoring cameras are now in use on many occasions. For example, monitoring cameras are used not only indoors for monitoring banks, convenience stores, and pachinko parlors but also outdoors for monitoring stations, roads, rivers, and dams. It is expected that the application fields of monitoring cameras will be broaden in the future.
The cost of monitoring cameras has greatly reduced. However, the installation of an unlimited number of monitoring cameras is not practical. In consideration of cost-effectiveness, desired image quality, etc., it is requested to cover as a wide monitoring area as possible with a limited number of cameras. When panning and tilting bases are not used, wide-viewing-angle lenses would be required.
In general, wide-viewing-angle lenses cause images to be distorted into barrel shapes. Distorted images are greatly different from landscapes actually seen by the human eyes. For example, in a distorted image, straight lines are not reflected as they are, and the size of an object is made different between the center and end of a screen. Thus, the distorted image gives a strong sense of discomfort, which influences the degree of fatigue of a person who monitors the camera image. Therefore, it is desired that such distortion be corrected.
Expensive lenses correct aberration such as distortion with their high refractive indexes, aspherical surfaces, and large number of groups (per lens). However, it is difficult to use such expensive lenses for monitoring in view of cost. Thus, distortion correction processing based on signal processing is required.
As a conventional image signal processing apparatus that corrects distortion, an apparatus illustrated in, e.g., Patent Literature 1 is known. FIG. 12 is a block diagram of the image signal processing apparatus illustrated in the official gazette. The image signal processing apparatus has: a coordinate conversion unit 102 that calculates, from the position of a pixel of a color image, a corresponding sampling coordinate on a color mosaic image corresponding to the position of the pixel of the color image having been subjected to deformation processing; a sampling unit 104 that interpolates and generates the value of a pixel at the sampling coordinate for each of a plurality of color planes obtained by decomposing the color mosaic image; and a color generation unit 106 that combines the interpolation values of the respective color planes with each other to generate the color image. The image signal processing apparatus finds, from a color mosaic image, the value of each pixel of a color image having been subjected to deformation processing as the value of a pixel at a sampling coordinate by interpolation calculation. Thus, with single interpolation calculation, the image signal processing apparatus is allowed to implement both color interpolation processing that generates a color image from a color mosaic image and the deformation processing of the color image.
In the image signal processing apparatus described above, a color filter array having complementary-color arrangement may be used. However, in a case where data of a complementary-color-field color-difference sequential system is to be processed by the image signal processing apparatus, coordinate conversion for each color plane by linear interpolation causes resolution to be seriously degraded. In particular, horizontal resolution is likely to be degraded by interpolation in a horizontal direction before luminance data is generated.
As for a vertical direction, interlace data is processed by the image signal processing apparatus for each color filter plane. More specifically, since linear interpolation is performed for each interlaced scanning data, the loopback of a frequency component in the vertical direction is likely to occur, which causes an image to be degraded.